README.error: add workarround for error 'build-client-full' does not support setting an external commonName.

README.error

@@ -1,3 +1,17 @@
+# ---
+# Error: Option conflict: 'build-client-full' does not support setting an external commonName
+# ---
+
+# workarround:
+#
+#     see: https://github.com/OpenVPN/easy-rsa/issues/717
+#
+#     EASYRSA_REQ_CN should probably be removed from vars.example.
+
+fqp_path_to_vars="/etc/openvpn/server/gw-ckubu/easy-rsa/vars"
+
+perl -i -n -p -e "s/^\s*(set_var\s+EASYRSA_REQ_CN.*)/#####\1/g" ${fqp_path_to_vars}
+
 # ---
 # Error: AEAD Decrypt error: bad packet ID
 # ---